Microalgae can be grown under autotrophic conditions where sunlight is used as an energy source and CO2 as a carbon source. In heterotrophic cultures, microalgae use organic matter by fermentation as a carbon and energy source. Whether in open systems or closed systems such as photobioreactors, it is necessary to use culture mediums that contain all the nutrients required for microalgae growth and reproduction. What does the growth chamber do? What are the different culture mediums used in the cultivation of microalgae? Are there differences in the culture environment between autotrophy and heterotrophy?
The role of culture medium in the growth of microalgae
The mediums used for the cultivation of microalgae are preparations in which the microorganisms can grow and multiply. To fulfil this essential role, they must meet the nutritional requirements of the algae being produced. They provide sufficient mineral ions and potentially growth factors as well as vitamins. In the case of a heterotrophic culture, a source of carbon and energy such as glucose is added.
As well as supplying nutrients, the culture media must have a pH as close as possible to the optimal pH for microalgae growth. Finally, their ionic strength must be adapted so as not to cause cell destruction.
Synthetic culture mediums have a precisely exactly known composition, both qualitatively and quantitatively, whereas empirical mediums are a little more random, as they depend on the raw materials used: sea water, carbon source in the case of heterotrophic production, etc.
The autotrophic culture mediums
In a microalgae culture in SALT photobioreactors, the carbon, which alone represents more or less 50% of the dry matter of the microalgae, is derived entirely from CO2 in bottles connected to the system. This is supplied to the culture according to the needs of the cultivated algae and the pH regulation.
A conventional culture medium used for microalgae cultures in a PBR must contain nitrogen, which is usually supplied in the form of ammonia, nitrate or urea. The choice of either source depends mainly on the species being cultivated.
Phosphorus is also an essential element. In some cases, and depending on the culture medium, it is added in excess, as it has the unfortunate tendency to form complexes with certain metal ions. In this case, the algae can no longer assimilate it.
A culture medium also contains essential trace elements. These are often metal ions such as magnesium, which is important for the formation of chlorophyll, the essential molecule for photosynthesis.
A classic culture medium used to cultivate microalgae in photobioreactors is Conway’s medium. It mainly contains salts for the supply of nitrogen and phosphorus as well as trace metals. It may contain vitamins B1 and B12.
Conway’s medium is not used pure but is usually diluted with water. With an efficient photobioreactor, this culture medium can be concentrated to a greater or lesser extent.
Before use, it is sterilised using an autoclave or by filtration through 0.45 µm and 0.22 µm filters.
Another medium that can be used for the cultivation of microalgae is BBM (Bold Basal Medium). It differs from Conway’s medium in the type of salts used in its preparation. Some microalgae also grow better in one type of medium than in others. The cultivation of some diatom-type marine microalgae requires the presence of silica in the culture medium.
Conway culture medium can be purchased as comes, but preparation is simple and relatively cheap.
The heterotrophic culture medium
Microalgae grown heterotrophically in fermenters are supplied with carbon substrate, which they must find in their culture medium. This carbon source can take a variety of forms such as glucose, lignocellulosic carbohydrates, or sugar-rich molasses.
It is important to regularly renew the carbon substrate by regular additions to maintain the fermentation process at its best level.
The other elements, which are essential for the growth of microalgae, such as nitrogen, phosphorus and various microelements, are provided by salts present in the culture medium such as Conway’s medium.
The culture medium for the cultivation of microalgae is one of the fundamental elements for the production of excellent biomass.
Depending on the species of algae cultivated and the cultivation method, there may be differences. For example, in the case of large-scale algae cultivation in open raceway systems, the culture medium is usually either hypersaline or hyperalkaline to limit the risk of bacterial contamination. However, this may reduce the type of species that can be grown.